The present invention relates to clearance control in a turbine, such as a gas turbine.
Clearance in a turbine typically refers to the dimension of gaps between the rotor and the stator casing that surrounds the rotor. The rotor is typically an axial turbine having rows of buckets each mounted on a turbine wheel. The wheels are mounted on a shaft of the turbine. The stator casing houses the rotor and includes rows of stationary nozzles positioned between the rows of buckets. Clearance often refers to the annular gap between the tips of the buckets and the stator casing.
Clearance is needed to allow the buckets to rotate without rubbing against the stator casing. If the clearance is too great, combustion gases may leak over the tips of the buckets and do not drive the rotation of the turbine. If the clearance is too small, the tips may rub against the stator casing and may cause vibrations that damage the turbine.
The clearance varies as the turbine is heated and cools during its various operational phases. The variations in clearance are due to thermal expansion and contraction of the components of a turbine. A turbine is typically formed of metal components having different heat expansion rates. The turbine wheels, buckets on the wheels and annular shells around the buckets expand and contract at different rates. Due to different rates of thermal expansion, clearance could increase or shrink as the gas turbine heats and cools.
Clearance is needed whenever the turbine buckets rotate including: while the turbine heats up during startup, as the gas turbine transitions from full speed no load (FSNL) operation to and during full speed full load (FSFL) operation, and as the turbine shuts down. Maintaining adequate clearance during any and all operational phases of a gas turbine is achieved by a clearance control system.
Clearance control systems and techniques provide adequate clearance during all phases of gas turbine operation. Conventional clearance control systems and techniques include cooling systems mounted on external skids adjacent the gas turbine, complex sensing and actuation systems for regulating cooling flow bled from the compressor and used for turbine cooling and external heat transfer systems to heat or cool the cooling air before it enters the turbine. Conventional clearance control systems and techniques tend to be active in that they adjust the amount of a cooling fluid flowing through the shell or buckets. Some active conventional clearance systems are actuated in response to a certain operating conditions, such as at pinch points which occur when clearance is at its narrowest. For example, heating cooling air may be added to the turbine casing to increase the thermal expansion of the casing and thus increase the clearance at a pinch point.
Active clearance control systems are often mechanically complex, expensive and require computer or hydraulic controllers. Passive clearance control systems do not require controllers and tend to be relatively mechanically simple and inexpensive. However, passive controllers typically do not have the ability to adjust the cooling capacity of the cooling gas fed to the turbine. Even in view of the conventional clearance control systems, there remains a long felt need for clearance control systems and techniques that are robust, economical, assure adequate clearance at all phases of gas turbine operation and avoid excessive clearances especially at steady operating phases such as FSFL.